1. Field of the Invention
The present invention relates to a toothed belt for power transmission, and more particularly to a power-transmission toothed belt for use in general industrial machineries. The toothed belt of the type concerned is used for transmitting power from one pulley to another rotated at low speeds under heavily-loaded conditions.
2. Description of the Related Art
A toothed belt for driving a camshaft of an automobile engine is used under the conditions where the low-temperature resistance, high-temperature resistance, flexing resistance and quietness are severely required. Currently, the toothed belt is produced by vulcanizing and pressing (or otherwise molding) a rubber composition containing, as major ingredients, hydrogenated nitrile rubber and zinc polymethacrylate after several additives are blended with the rubber composition.
On the other hand, toothed belts designed for use in general industrial machineries are mainly used for the purpose of transmitting power between two shafts and they operate at low speeds under heavily loaded conditions. Under such conditions, the high-temperature resistance and quietness, such as required in an application to automobile engines, are not necessary. More specifically, a heat resistance on the order of 80xc2x0 C. is sufficient, and a low-temperature resistance enough to withstand the room temperature is satisfactory. In addition, the flexing resistance is not highly demanded. In the manufacture of the toothed belts for general industrial machineries, urethane rubber is selected for this purpose as a main component because the urethane rubber can provide sufficient physical properties even when the rigidity thereof is increased within a certain range. The urethane rubber is vulcanized and pressed or otherwise molded into a toothed belt with a tensile member made of aromatic polyamide fibers being embedded in the urethane rubber.
The toothed belts designed for use in general industrial machineries may occasionally be used at high speeds. In this instance, a rubber portion forming teeth (hereinafter referred to as xe2x80x9ctooth rubberxe2x80x9d) of the toothed belts becomes soft due to high self-heating properties of the urethane rubber. As a consequence, the rigidity of tooth rubber decreases, tending to cause a breakage of teeth present on the toothed belts. This same teeth breakage problem may also occur when the toothed belts are used under high temperature environmental conditions.
Thus rubber compositions are used for automobile engines to produce a toothed belt for general industrial machineries. However, since the conventional toothed belts for automobile engines are designed for use under relatively low load conditions, if components of the rubber composition are changed to withstand the heavily loaded conditions, the high-temperature resistance, low-temperature resistance, flexing resistance and quietness are lost.
In addition, since aromatic polyamide fibers are used in making the tensile member of the conventional toothed belts, the dimensional stability of the tensile member is relatively low because the fibers are likely to undergo elongation and contraction when subjected to changes in the temperature, humidity or the like environmental condition. Use of such dimensionally unstable tensile members brings about remarkable changes in the tooth-belt tension, leading to reduction of the lifetimes of the toothed belts and peripheral devices as well as the generation of unpleasant operation noises.
Furthermore, since the aromatic polyamide fibers poorly adhere to the urethane rubber, a peel-off problem of the tooth rubber may arise. In addition, it is difficult to improve the bonding strength between the fibers. Thus, in heavily loaded, high-speed applications, the toothed belt encounters a tooth-breakage problem as well as a breakage of the toothed belt.
It is therefore an object of the present invention to provide a toothed belt having qualities suitable for use in general industrial machineries.
Another object of the present invention is to provide a toothed belt formed from a rubber composition which is capable of withstanding heavily loaded, high-speed conditions.
A further object of the present invention is to provide a toothed belt formed from a rubber composition which has a formulation improved to increase the lifetime of the toothed belt and lower operation noises of the toothed belt.
Still another object of the present invention is to improve the affinity between a rubber composition and a tensile member of the tooth belt thereby increasing the bonding strength a between rubber and the tensile member to such an extent that the tooth rubber peel-off problem from the toothed belt is completely prevented.
In one aspect, the present invention provides a toothed belt comprising a cured rubber belt body formed primarily from a vulcanizable rubber composition containing hydrogenated nitrile rubber and zinc polymethacrylate, and a tensile member embedded in the belt body, wherein the rubber composition is comprised of zinc polymethacrylate and hydrogenated nitrile rubber having an iodine value of 4-28 g which are blended together in a ratio of 22:78 to 45:55 parts by weight.
In a toothed belt having a belt body formed from a vulcanizable rubber composition containing hydrogenated nitrile rubber and zinc polymethacrylate, the zinc polymethacrylate forms a higher-order network structure in a polymer of the belt body. Accordingly, a proportion of the zinc polymethacrylate must be held down to some extent; otherwise, in an application where the toothed belt is driven to run at high speeds along a complicated path, the toothed belt would encounter a problem that the flexing resistance is remarkably reduced and the belt body separates or peels off from the tensile member.
However, since the general industrial machineries require heavy duty power transmission, toothed belts used therein operate at relatively low speeds because they are mainly used for the purpose of achieving power transmission between two shafts. In the present invention, this particular use or application is perceived, and the aforesaid higher-order network structure forming-function of the zinc polymethacrylate is utilized so that a rubber composition containing hydrogenated nitrile rubber and zinc polymethacrylate can possess an on-load performance (load-bearing properties) which is improved to the level of urethane rubber. The on-load performance is particularly chosen from among many performances of the conventional toothed belts required for use in the general industrial machineries.
To improve the on-load performance, the present invention sets the blending ratio of zinc polymethacrylate and hydrogenated nitrile rubber to be in a range of 22:78 to 45:55 parts by weights. If the proportion of zinc polymethacrylate is below 22 parts by weight, a belt body formed after vulcanization of the rubber composition achieves an insufficient on-load performance and, hence, the belt body is not suitable for use in the general industrial machineries requiring heavy duty power transmission. Conversely, if the proportion of zinc polymethacrylate exceeds 45 parts by weight, the hardness of the belt body is increased to a level which makes the flexing resistance and quietness of the toothed belt unsatisfactory even in an application to the general industrial machineries.
Preferred examples of the hydrogenated nitrile rubber used in the present invention include hydrogenated nitrile rubber with a nitrile content of 30 to 50% and having an iodine value of 4-28 g. These examples include a 90-98% hydrogenated nitrile rubber which is available in the market (under the product name xe2x80x9cZ-pole seriesxe2x80x9d of Nippon Zeon Co, Ltd.
If the iodine value is not greater than 4 g, a great amount of vulcanizing agent added to provide prescribed rubber properties exerts a negative influence to the resorcin-formalin-latex of the tensile member (core) such that an unnecessarily great rigidity is provided to the toothed belt, tending to lower the flexural fatigue resistance of the toothed belt. Conversely, if the iodine value exceeds 28 g, an attempt to provide the prescribed rubber properties will deteriorate the heat resistance and ozone resistance and thereby lower the effect of the hydrogenated nitrile rubber to such an extent that the use of normal nitrile rubber is rather economical.
The rubber composition is preferably formed of a polymer alloy composite material and hydrogenated nitrile rubber having an iodine value of 4-28 g which are blended together in a ratio of 55:45 to 90:10 parts by weight. The polymer alloy composite material is formed of zinc polymethacrylate and hydrogenated nitrile rubber having an iodine value of 4-28 g which are dispersed in a ratio of 40:60 to 50:50 parts by weight.
The polymer alloy composite material is prepared, then the hydrogenated nitrile rubber is added to the polymer alloy composite material, so that before vulcanization is carried out, the zinc polymethacrylate can be homogeneously dispersed in the hydrogenated nitrile rubber, leading to formation of a cured rubber belt body having a fine, higher-order network structure. With this process, an on-load performance can be obtained which is more suitable for use in the general industrial machineries than that obtained by a process in which hydrogenated nitrile rubber and zinc polymethacrylate are initially blended together.
In another aspect, the present invention provides a toothed belt comprising a cured rubber belt body formed primarily from a vulcanizable rubber composition containing nitrile rubber having a Mooney viscosity of 30-90 at 100xc2x0 C. and zinc polymethacrylate, and a tensile member embedded in the belt body, wherein the nitrile rubber and the zinc polymethacrylate are blended together in a ratio of 100:20 to 100:60 parts by weight.
The hydrogenated nitrile rubber is relatively expensive. In view of this, the nitrile rubber is used in place of the hydrogenated nitrile rubber. With respect to the heat resistance and ozone resistance, the nitrile rubber is inferior to the hydrogenated nitrile rubber but use of the nitrile rubber can avoid over-quality when the toothed belt is used under low-temperature environmental conditions not accompanying generation of ozone at high densities.
Use of the zinc polymethacrylate greatly deteriorates the productively of the rubber composition and properties of the rubber composition before the vulcanization is carried out. To deal with this problem, the present invention uses the nitrile rubber having a Mooney viscosity of 30-90 at 100xc2x0 C. in combination with carbon black.
It the Mooney viscosity is below 30, a great amount of carbon black must be added to provide the predetermined productively and pre-vulcanization properties of the rubber composition, failing to provide the desired properties when the vulcanization is carried out. On the contrary, if the Mooney viscosity exceeds 90, the desired properties can obtained by a small amount of carbon black added to the nitrile rubber but it is still difficult to obtain the prescribed productivity and pre-vulcanization properties of the rubber composition.
In addition, if the proportion of the zinc polymethacrylate, based on 100 parts by weight of nitrile rubber, is less than 20 parts by weight, the cured (vulcanized) rubber belt body exhibits an insufficient on-load performance. On the other hand, if the proportion of the zinc polymethacrylate, based on 100 parts by weight of nitrile rubber, exceeds 60 parts by weight, the hardness of the cured rubber belt body goes up and hence deteriorates the flexibility of the belt body. This may lead to a considerable reduction of the lifetime of the toothed belt and a generation of increased noise during operation of the toothed belt.
The present invention utilizes high-strength glass fibers as the tensile member because the high-strength glass fibers exhibit a little dimensional change against changes in surrounding conditions and hence can stabilize a tension in the toothed bent while in use. The high-strength glass fibers thus used are effective to elongate the lifetime of the toothed belt and suppress the generation of operation noises while the toothed belt is running.
It is preferable that the tensile member has a resorcin-formalin-latex layer and an overcoat layer both formed by a pre-treatment. The tensile member treated thusly exhibits good adhesion to the rubber tooth body and is free from separation from the rubber tooth body. In addition, the latex and overcoat layers prevent breakage of the fibers caused by the rubbing between the fibers and thereby improves the integrity between the fibers as well as the integrity between adjacent cords formed by the high-strength glass fibers and jointly forming the tensile member. Thus, even when the hardness of the cured rubber belt body becomes high, the reduction of the flexural fatigue resistance of the toothed belt is very small. Accordingly, the toothed belt as a whole has a prolonged service life.